In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of conductors (hereinafter “wire pair” or “conductor pair” or “differential pair”) rather than over a single conductor. The conductors may comprise, for example, wires, contacts, wiring board traces, conductive vias, other electrically conductive elements and/or combinations thereof. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors. This transmission technique is generally referred to as “balanced” transmission.
When a signal is transmitted over a conductor, electrical noise from external sources such as lightning, electronic equipment and devices, automobile spark plugs, radio stations, etc. may be picked up by the conductor, degrading the quality of the signal carried by the conductor. With balanced transmission techniques, each conductor in a differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, and thus the noise signal may be substantially cancelled out by the subtraction process.
Many communications systems include a plurality of differential pairs. For example, the typical telephone line includes two differential pairs (i.e., a total of four conductors). Similarly, high speed communications systems that are used to connect computers and/or other processing devices to local area networks and/or to external networks such as the Internet typically include four differential pairs. In such systems, channels are formed by cascading plugs, jacks and cable segments (herein, a “channel” refers to the end-to-end connection for the four differential pairs that connect one end device to another end device). In these channels, when a plug mates with a jack, the proximities and routings of the conductors and contacting structures within the jack and/or plug can produce capacitive and/or inductive couplings. Moreover, in the cable segments of these channels four differential pairs are usually bundled together within a single cable, and thus additional capacitive and/or inductive coupling may occur between the differential pairs in each cable. These capacitive and inductive couplings give rise to another type of noise that is called “crosstalk.”
“Crosstalk” in a communication system refers to an unwanted signal that appears on the conductors of an “idle” or “victim” differential pair that is induced by a disturbing differential pair. “Crosstalk” includes both near-end crosstalk, or “NEXT”, which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), as well as far-end crosstalk, or “FEXT”, which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both NEXT and FEXT are undesirable signals that interfere with the information signal.
A “disturbing” differential pair may impart two different types of crosstalk onto another differential pair. The nature of the induced voltage determines which of two types of crosstalk is occurring. The first of these two types of crosstalk is referred to as differential-to-differential crosstalk (XTLKDD). It occurs when the induced voltages from the source differential pair that are imparted on both the conductors of the victim differential pair are unequal. Differential-to-differential crosstalk is measured as the ratio of the induced differential voltage on the victim pair to the source or driven differential voltage on the disturbing pair (typically referenced as 1 volt). Differential voltage is defined as the difference between the voltages on the two conductors of the differential pair, i.e., Vdiff=(V1-V2, where V1 is the voltage on conductor 1 and V2 is the voltage on conductor 2 of the differential pair. Differential-to-differential crosstalk is typically expressed in decibels (dBs) and can be defined as:XTLKDD=20 log(V1-V2)where V1 is the induced voltage on conductor 1 of the victim pair and V2 is the induced voltage on conductor 2 of the victim pair.
The second of the two types of crosstalk is referred to as differential-to-common mode crosstalk (XTLKDC). Differential-to-common mode crosstalk occurs when the induced voltage is common to both conductors of the victim differential pair, and hence the victim pair can be viewed as being a single conductor. The voltage that is common to both conductors is called the common mode voltage (VCM) and is expressed as the average voltage on the two conductors of the differential pair, i.e., VCM=(V1+V2)/2. Differential-to-common mode crosstalk is measured as the ratio of the induced common mode voltage on the victim differential pair to the source or driven differential voltage of the disturbing pair. It is also expressed in dBs as:XTLKDC=20 log((V1+V2)/2)where V1 and V2 are as described above. Note that the voltages V1 and V2 can be calculated from the inductive and capacitive coupling parameters between disturbing and victim conductors. Further note that if V1=−V2, then VCM=0 and differential-to-common mode crosstalk is zero. Under this condition, the circuits are considered balanced. This is a desirable condition to minimize a type of crosstalk known as “alien NEXT” (which is described in more detail herein) in the channel.
A variety of techniques may be used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors (which are typically insulated copper wires) in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the conductors of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two conductors of a differential pair onto the other conductors in the cable tends to cancel out.
While such twisting of the conductors and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors (i.e., jacks and plugs) that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the jack and plug configurations that were adopted years ago generally did not maintain the conductors of each differential pair a uniform distance from the conductors of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already in place in existing homes and office buildings, the connector configurations have, for the most part, not been changed. As such, the conductors of each differential pair tend to induce unequal amounts of crosstalk on each of the other conductor pairs in current and pre-existing connectors. As a result, many current connector designs generally introduce some amount of NEXT and FEXT crosstalk.
Pursuant to certain industry standards (e.g., the TIA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association), each jack, plug and cable segment in a communications system may include a total of eight conductors 1-8 that comprise four differential pairs. By convention, the conductors of each differential pair are often referred to as a “tip” conductor and a “ring” conductor. The industry standards specify that, in at least the connection region where the contacts (blades) of a modular plug mate with the contacts of the modular jack (i.e., the plug-jack mating point), the eight conductors are aligned in a row, with the four differential pairs specified as depicted in FIG. 1. As known to those of skill in the art, under the TIA/EIA 568, type B configuration, conductor 5 in FIG. 1 comprises the tip conductor of pair 1, conductor 4 comprises the ring conductor of pair 1, conductor 1 comprises the tip conductor of pair 2, conductor 2 comprises the ring conductor of pair 2, conductor 3 comprises the tip conductor of pair 3, conductor 6 comprises the ring conductor of pair 3, conductor 7 comprises the tip conductor of pair 4, and conductor 8 comprises the ring conductor of pair 4.
As shown in FIG. 1, in the connection region where the contacts (blades) of a modular plug mate with the contacts of the modular jack, the conductors of the differential pairs are not equidistant from the conductors of the other differential pairs. By way of example, conductor 2 (i.e., the ring conductor of pair 2) is closer to conductor 3 (i.e., the tip conductor of pair 3) than is conductor 1 (i.e., the tip conductor of pair 2) to conductor 3. Consequently, differential capacitive and/or inductive coupling occurs between the conductors of pairs 2 and 3 that generate both NEXT and FEXT. Similar differential coupling occurs with respect to the other differential pairs in the modular plug and the modular jack.
U.S. Pat. No. 5,997,358 to Adriaenssens et al. (hereinafter “the '358 patent”) describes multi-stage schemes for compensating NEXT for a plug-jack combination. The entire contents of the '358 patent are hereby incorporated herein by reference as if set forth fully herein. The connectors described in the '358 patent can reduce the “offending” NEXT that may be induced from the conductors of a first differential pair onto the conductors of a second differential pair in, for example, the contact region where the blades of a modular plug mate with the contacts of a modular jack. Pursuant to the teachings of the '358 patent, a “compensating” crosstalk may be deliberately added, usually in the jack, that reduces or substantially cancels the offending crosstalk at the frequencies of interest. The compensating crosstalk can be designed into the lead frame wires of the jack and/or into a printed wiring board that is electrically connected to the lead frame within the jack. As discussed in the '358 patent, two or more stages of NEXT compensation may be provided, where the magnitude and phase of the compensating crosstalk signal induced by each stage, when combined with the compensating crosstalk signals from the other stages, provide a composite compensating crosstalk signal that substantially cancels the offending crosstalk signal over a frequency range of interest. The multi-stage (i.e., two or more) compensation schemes disclosed in the '358 patent can be more efficient at reducing the NEXT than schemes in which the compensation is added at a single stage, especially when the second and subsequent stages of compensation include a time delay that is selected and/or controlled to account for differences in phase between the offending and compensating crosstalk signals. Efficiency of crosstalk compensation is increased if the first stage or a portion of the first stage design is contained in the lead frame wires.
Another type of crosstalk that must be considered is “alien” crosstalk and, in particular, alien NEXT. Alien NEXT is the differential crosstalk that occurs between communication channels. Obviously, physical separation between the jacks of the two channels at issue helps reduce alien crosstalk levels, as may some conventional crosstalk compensation techniques. However, a problem case may be “pair 3” of one channel crosstalking to “pair 3” of another channel, even if the pair 3 plug and jack wires in each channel are remote from each other and the only coupling occurs between the routed cabling. This form of alien NEXT occurs because of pair-to-pair unbalances that exist in the plug-jack combination, which results in mode conversions from differential NEXT to common mode NEXT and vice versa. In particular, differential-to-common mode crosstalk from pair 3 to both pair 2 and pair 4 can contribute to such mode conversion problems. To reduce this form of alien NEXT, shielded systems containing shielded twisted pairs or foiled twisted pair configurations may be used. However, the inclusion of shields can increase cost of the system. Another approach to reduce or minimize alien NEXT utilizes spatial separation of cables within a channel and/or spatial separation between the jacks in a channel. However, this is typically impractical because bundling of cables and patch cords is common practice due to “real estate” constraints and ease of wire management.